Nuclear magnetic resonance (NMR) logging is a type of well logging that uses the NMR response of a formation to directly determine its porosity and permeability. NMR logging exploits the magnetic moment of hydrogen, which is abundant in rocks in the form of water. The NMR signal amplitude is proportional to the quantity of hydrogen nuclei present in a formation and can be calibrated to give a value for porosity that is free from lithology effects.
NMR logs provide information about the quantities of fluids present in a geological formation, the properties of these fluids, and the sizes of the pores containing these fluids. From this information, it is possible to infer or estimate the volume (porosity) and distribution (permeability) of the rock pore space, the rock composition, the type and quantity of fluid hydrocarbons, as well as the hydrocarbon producibility.
Generally, NMR tools operate by using an imposed magnetic field. This magnetic field is traditionally referred to as the “main magnetic field” or the “static field” as it is usually independent of time and is given the symbol B0. A second magnetic field, which varies in time, is also applied. This field is designated as B1 and is traditionally called the “radio frequency (RF) field”. It is turned on and off at different increments, known as a pulse. This perturbing field is usually applied in the form of an RF electromagnetic pulse whose useful magnetic component, B1, is perpendicular to the static field, B0. The perturbing field moves the orientation of the magnetization into the transverse (perpendicular) plane. The frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually re-polarizing to align with the static field while losing coherence in the transverse plane. The precessing nuclei generate a detectable RF signal that can be used to measure statistical distributions that can be converted into measurements of porosity (i.e., the relative amount of void space in the formation), hydrocarbon saturation (i.e., the relative percentage of hydrocarbons and water in the formation fluid), and permeability (i.e., the ability of formation fluid to flow from the formation into the wellbore).
When the B1 magnetic field is created by an RF coil powered by a transmitter, ringing, resonance, and other artifacts of this conventional system may reduce the accuracy and resolution of NMR measurements.